I think that it's tied to a very primitive, instinctive perception of things. Glass floors are a good example. I understand that they're far more than strong enough to support my weight. Far more than my weight. I've seen the numbers. I understand them. I understand that there is no risk. But I can't walk on them. The thinking part of my mind understands that this is a solid floor that's far more than strong enough and there is no risk in walking over it. The instinctive part of my brain is screaming "HOLE. DANGER. FALL. DEATH. AVOID." and it wins. I think that the situation you describe comes about because the height of a ladder, bridge or building is within the range of your instinctive perception of height but a plane at normal parachuting height is above it so there isn't the instinctive "height = danger" response.
In my case, it's also linked to a bizarre urge to jump off. I've no idea why. It's not suicidal thoughts. Not any more. I understand the difference. Better than I'd like to. It's this completely irrational and inexplicable urge to jump for no reason. I can comfortably tolerate a couple of metres. I can uncomfortably tolerate maybe 5 metres if I'm standing on something solid and holding onto something solid and there's a barrier between me and the drop. I'm OK looking out of a window in a building, though I wouldn't be if the window was open and I was looking down. I did a sort of DIY exposure therapy by forcing myself to walk over a pedestrian bridge over a road, repeatedly. I don't like it, but I can do it now without sweating with the effort of forcing down the panic. Since it doesn't adversely affect my life, I'm fine with it.
